In recent years, farm tractors have been provided with engines of substantial horsepower that permit them to be used regularly for transporting bulk materials from place to place at higher speeds. This bulk material is usually carried in a bulk carrier, trailer, wagon or other implement that is attached to a drawbar on the back of the tractor. These and similar devices supported by wheels and towed behind the tractor by a tongue are called “implements” herein.
Historically, agricultural tractors were equipped with a drawbar, a long slender member coupled to the chassis of the tractor and extending backward between the two larger wheels to a point at which it could be coupled to the implement's tongue. In the traditional design, the drawbar consists of an elongate steel bar having a hole at the rear end in which a hitch pin can be inserted to couple the implement tongue to the drawbar. The drawbar also had a hole in the forward end through which a bolt is passed to attach the drawbar to the chassis of the tractor.
As horsepower increased on tractors, however, the stress and strain on the pin coupling the implement tongue to the drawbar became a problem. The pins tended to wear and break under the high shear forces applied to the implement tongue.
One effort to prevent this problem was to provide an add-on reinforcing strap that could be bolted to the top surface of the drawbar near its rearward end. This add-on strap was typically made of the same bar stock from which the drawbar was made. It had an “S”-bend in the middle such that when the forward end was bolted to the drawbar, the rear end of the add-on strap was parallel to end spaced apart from the drawbar, forming a gap therebetween in which the tongue of the implement could be inserted.
The drawbar and the add-on strap (also called a “hammer strap” or “clevis”), had holes therethrough through which a pin could be inserted first to the hammer strap, then through the tongue of the implement, and then into the drawbar itself. By supporting the pin on each end, the drawbar with hammer strap distributed the implement load on the tongue to two holes, and not just one through the drawbar. This permitted the drawbar to be used to tow much larger loads with much less wear and fatigue. An example of one of these early drawbars can be seen in U.S. Pat. No. 2,654,613, which issued to Blair et al. on Mar. 28, 1952. This solution, however, was not without its own problems. The drawbar was substantially weakened by stress concentrations formed when the holes necessary for attaching the hammer strap to the drawbar were drilled through the drawbar.
As tractor horsepower has further increased, the tongue loads applied to drawbars have continued to increase. The previous solutions are inadequate and new structures for stiffening or reinforcing the drawbar, hammer strap and drawbar hanger are needed.